>That's correct Ian. ..snip....
>Eimac usually give a grid and screen voltage that provides operation
>in an optimum area of voltage ratios. While you can "fudge" around a
>bit, Rich's carte blanche suggestion of picking voltages to adjust
>drive might very well move the tube into problem operating areas.
>
.. Did Rich say that the screen can be operated above its max. V rating?
>For example, if I had a 4CX1000A with a 50 ohm grid resistor and used
>Rich's method with a FT1000, the tube would be operated far outside
>optimum bias and screen voltages for normal quiescent current. The
>peak RF grid voltage would be 140 volts, requiring 140 volts of bias.
>
It looks like I'm going to have to draw Tom a map. ....... Here's
Rich's method. The needed bias for a 4CX1000A is 70 V with X volts on
the screen and Y volts on the anode. However, the driver produces 140
peak V. This means that we need to halve the 140V RF drive voltage. To
do this we use a bifilar (4 to 1) stepdown toroidial transformer. The
requisite grid termination resistor is 12.5 ohms. The exciter sees 50
ohms. The grid sees 70 peak V. Everybody's happy. If there's a twinge
of dreaded grid current on voice peaks, adjust the grid bias V higher
until there is zero grid current. Reset the ZSAC with the screen V
adjustment. Peak C1 and load with C2 until you see about 5mA of screen
current. The End. ...... ...... It sounds simple, because it is.
- - Notes:
1. Due to the reversing screen current characteristic of this tetrode,
the screen V should probably be obtained from a series string of 15v
-20v, 5w zeners. Screen V can be controlled with a 5-pos. rotary switch
that progressively shorts zeners. Max. rated screen V is 400. To
safely limit screen current, zener bias should be obtained through a
resistor connected to the anode supply. If the anode supply fails, the
screen V disappears. If the anode supply has a bleeder resistor, no
shunt resistor is needed across the zener string.
2. A method of increasing the drive requirement in a smaller increment
is to add an approx. 5-ohm low-L resistor between the cathode and ground
in order to provide RF-NFB. With 320 screen V, and 0 grid V, the peak
cathode current for the 4CX1000A is about 3A, so -- such a resistor
increases the drive requirement by approx. 15 peak volts.
3. In normal ambient temperatures, an uncrowded '5w' rated zener diode,
with full lead lengths, can safely dissipate only about 2w. Forced air
cooling increases MTBF.
4. Selection of the resistor between the zener string and the anode
supply deserves close attention. (In the above case the voltage drop is
about 3000v -400v = 2600v at a current of about 35mA.) Power resistors
have max. voltage ratings as well as max. power ratings. For high R
wirewound resistors, using a resistor at the stated power rating may
violate the maximum voltage rating. Connecting two or more resistors in
series, to divy up the voltage drop, helps.
5. The 4CX1000A has a maximum grid dissipation rating of 0W!!. With the
anode cooler removed, the 4CX1000A's grid wires are so delicate they are
virtually impossible to see without magnification. To limit grid
current, I would use a grid supply whose output R was about 100k ohms.
The naysayer-recommended "stiff" grid voltage supply could make a stiff
blow to your wallet.
Rich---
R. L. Measures, 805-386-3734, AG6K
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